As part of the central nervous system, both developmentally and phenotypically, the retina shares the recalcitrance of brain and spinal cord with respect to functional repair. This is unfortunate in that, among heritable conditions alone, there are over 100 examples of diseases involving the loss of retinal neurons. One strategy for replacing these cells has been to transplant retinal tissue from healthy donors to the retina of the diseased host. While the results of such studies have been encouraging in terms of graft survival, the problem of integration between graft and host has proved daunting.
Studies of retinal development have been possible, using fetal human retinal cell cultures (e.g., Kelley et al.10). However, such cultured cells are not stem or progenitor cells as they lack the multipotency characteristic of ture stem cells. The recent isolation and amplification of multipotent stem cells (variously referred to as progenitor cells, immature cells, undifferentiated cells, or proliferative cells), in a laboratory setting1,2 has enlivened the fields of mammalian development and transplantation. Some have shown that examples of these proliferative, stem or progenitor cells, present in the adult rodent hippocampus, can be isolated, cultured and transplanted into various sites within the central nervous system (CNS), where they can differentiate into neurons or glial cells. It has also been shown that transplanted adult hippocampal progenitor cells are able to migrate into, and differentiate within, the mature dystrophic retina. However, the isolation of true stem cells from the neuroretina, particularly ones able to differentiate into functional photoreceptor cells both in vitro and in vivo, has proven elusive.
Recently, van der Kooy et al., in U.S. Pat. No. 6,117,675 (September 2000), have described a putative “retinal stem cell” derived from the ciliary marginal zone pigment epithelial layer, which cell is not found in neuroretina, is pigmented, is nestin-negative, and can proliferate and be passed in the absence of any growth factor. Such pigmentation, nestin negativity, and non-reliance on growth factors are unusual for mammalian stem cells. As well, van der Kooy et al. provide no evidence of the ability of their putative retinal stem cells to integrate into a host retina and to differentiate into functional mature cells, in vivo.
Additionally, the very plasticity that makes stem cells so interesting biologically, makes them difficult to track as they integrate into host tissues.
Therefore, there remains a need for multipotent, neuroretinal stem cells that can be amplified ex vivo and that readily differentiate into photoreceptor cells following transplantation to the eye, which is met by the present invention. The present invention also provides a method of tracking these cells when introduced into a host organism.